1. Field of the Invention
The present invention relates to a linear bearing compensation system, more particularly to a linear bearing compensator utilizing spherical bearings to isolate a load or carriage plate from alignment errors between parallel sets of linear motion bearings.
2. State of the Related Art
Linear bearing systems are well known in the mechanical art and are used wherever there is a need to minimize friction between two surfaces along a linear path. A linear bearing system is typically comprised of a track or rail member and a slider block mounted on and retained about the track or rail. See, for example, U.S. Pat. Nos. 5,158,372 and 4,527,840. The track member is comprised of a generally rigid, linear, precision machined metal rail having a consistent cross-section design and capable of supporting heavy loads. The slider block is generally comprised of a machined metal housing which fits over the track, means for retaining the slider block about the track, bearing means within the housing interposed between the slider block and the track, a lubrication port and a lubricant retention system. The lubrication system lubricates the bearing means and the metal to metal interfaces between the slider unit and the track, thereby decreasing the rolling friction between the slider block and the track member. The top of the slider block is often drilled and tapped to accommodate retaining means for various loads placed atop the slider block. The particular cross section of the track and slider block, as well as the bearing and lubrication means will vary from manufacturer to manufacturer.
Linear motion systems often utilize two parallel linear bearings mounted on a floor or structural framework. The parallel tracks are designated as the master and the slave tracks. The master track is utilized as a positional and dimensional reference for the slave track. A carriage plate is to be retained on two or more slider blocks with various loads adapted to fit on the carriage plate. Alternatively, a load may be mounted directly on the slider blocks themselves. A linear motion system further includes a positioning means to index the slider blocks and/or carriage plate along the tracks. The positioning means includes a drive means, such as a machine screw system, chain system, direct drive rack and pinion system or various other known drive means. The positioning means further includes a positioning verification system which assures that the slider blocks and load are positioned at the desired location on the track.
The linear bearing tracks and slider blocks are machined to a relatively tight tolerances. However, the parallel tracks are often mounted on a floor or within a structural framework which is not constructed or manufactured to the same degree of precision. The cost of manufacturing a precision surface on which to mount the tracks may be justified in some instances, such as precision machine tools, but is often not cost justifiable in other system. For example, material handling systems are often mounted on overhead gantries or directly on a cement floors. It would be impractical, and not cost justifiable to construct a gantry support system for the rails having the required precision positioning and dimensional stability, or constructing a concrete floor that is flat, level and has the necessary smoothness to match that of the rail itself.
FIGS. 1A-1F illustrate the various types of alignment errors which may be introduced into a linear motion system as the result of mounting errors. FIG. 1A is a cross sectional view of a master track M and a slave track S, in which the slave track S is not mounted at the same height or level as the master track M. It will be appreciated that the non-level tracks will introduce an alignment error E into the linear motion system. This alignment error will result in additional mechanical forces being introduced into the system. Further, it may result in positioning errors.
A second type of alignment error is illustrated in FIG. 1B. In FIG. 1B, the slave track S is not mounted parallel to the master track M. The master M and slave S tracks are mounted a nominal distance D apart and the non-parallel condition introduces an error E such that the master M and slave S tracks are a distance D+E apart further down the tracks. Movement along the tracks by slider units and the carriage plate will introduce angular errors and external mechanical forces into the linear motion system.
A third type of alignment error is illustrated in FIG. 1C. In FIG. 1C, the ends of slave track S are shown as being spaced a consistent distance D from the master track M. However, the slave track S is mounted in a non-linear fashion relative to master track M, introducing a lateral error E in the track. This type of alignment error may occur when the retaining holes for the slave track S are not drilled parallel to master track M, resulting in a warpage in the slave track S when it is bolted to the framework or floor. This type of error will also introduce dimensional errors into the system and additional mechanical resistance.
A fourth type of alignment error is illustrated in FIGS. 1D and 1E. In FIG. 1D, the master M and slave S tracks appear to be mounted parallel to each other. However, the same tracks are depicted in FIG. 1E, which is a side view of FIG. 1D. As may be seen in FIG. 1E, the slave track S varies in height down its length, introducing vertical error E into the system. This type of error may result from a non-level floor or framework mounting surface.
A fifth type of alignment error is illustrated in FIG. 1F. In FIG. 1F, the vertical axis of slave track S is not parallel with the master track M vertical axis. Thus, while the bottom of slave track S is the desired nominal distance D from the master track M, the top of slave track S is a distance D+error E from the master track M.
FIGS. 1A-1F are illustrative of the types of alignment errors which may occur when mounting parallel linear bearing tracks. It will be appreciated that while each type of error has been shown in isolation in FIGS. 1A-1F, combinations of these errors may occur as the tracks are secured to a framework or floor.
The prior art discloses a number of linear bearing compensators intended to overcome these various errors. These prior art devices include U.S. Pat. No. 4,995,734 to Schroeder and U.S. Pat. No. 4,637,738 to Barkley. These systems attempt to compensate for alignment errors by introducing a compensation element having a limited range of movement. However, the alignment compensators in these systems are limited in the degrees of freedom and introduce mechanical forces into the linear motion system as a result of their compensation for alignment errors. It will be appreciated that the introduction of external mechanical forces to the linear motion system will result in additional wear on the bearing systems and the linear drive system.
Thus, there exists a need for a low cost linear bearing compensator which permits a high degree of freedom without introducing external forces into the linear motion system.